CART2014
Photosynthesis converts absorbed solar energy to a protonmotive force, which drives ATP
synthesis. The membrane network of chlorophyllâprotein complexes responsible for light
absorption, photochemistry and quinol (QH) production has been mapped in the
purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides using atomic force
microscopy (AFM), but the membrane location of the cytochrome bc (cyt
bc) complexes that oxidise QH to quinone (Q) to generate a protonmotive
force is unknown. We labelled cytbc complexes with gold nanobeads, each
attached by a Histidine (His)-tag to the C-terminus of cytc.
Electron microscopy (EM) of negatively stained chromatophore vesicles showed that the
majority of the cytbc complexes occur as dimers in the membrane. The cyt
bc complexes appeared to be adjacent to reaction centre light-harvesting 1-PufX
(RC-LH1-PufX) complexes, consistent with AFM topographs of a gold-labelled membrane.
His-tagged cytbc complexes were retrieved from chromatophores partially
solubilised by detergent; RC-LH1-PufX complexes tended to co-purify with cyt
bc, whereas LH2 complexes became detached, consistent with clusters of cyt
bc complexes close to RC-LH1-PufX arrays, but not with a fixed, stoichiometric
cytc-RC-LH1-PufX supercomplex. This information was combined with a
quantitative mass spectrometry (MS) analysis of the RC, cytbc, ATP synthase,
cytaa and cytbb membrane protein complexes, to construct an
atomic-level model of a chromatophore vesicle comprising 67 LH2 complexes, 11 LH1-
RC-PufX dimers & 2 RC-LH1-PufX monomers, 4 cytbc dimers and 2 ATP
synthases. Simulation of the interconnected energy, electron and proton transfer processes
showed a half-maximal ATP turnover rate for a light intensity equivalent to only 1% of
bright sunlight. Thus, the photosystem architecture of the chromatophore is optimised for
growth at low light intensities.

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